1. Field of the Invention
The present invention relates to a transmission system which connects communication devices with a transmission path and a receiving path, and performs bi-directional data communication synchronizing the transmission path and the receiving path.
2. Description of Related Art
In a transmission system which performs bi-directional data communication, transmission quality may deteriorate on transmission paths. A transmission error is an example of transmission deterioration. Quality deterioration, such as a transmission error, is caused, for example, by an age-based change of characteristics of an amplifier and a timing regeneration circuit equipped on a transmission path, and the alteration of transmission data caused by an accident.
An operator of a transmission system must maintain the transmission system by constantly detecting the degree of transmission errors.
Parity check is one method to detect a transmission error. In parity check, a "0" or "1" parity bit is added to transmission signals to regulate the transmission signals, and a communication device at the receive side detects a transmission error by judging whether this regularity is maintained after transmission.
Another method to detect a transmission error is to return data from the communication device at the receive side to the communication device at the transmission side, comparing the transmission signals and returned signals by the communication device at the transmission side.
In parity check, the communication device at the receive side can detect transmission quality in real-time, however the communication device at the transmission side cannot detect the transmission quality. That is, with known methods, it is impossible for the communication device at one end to detect transmission quality of both the transmission path and receiving path.
The method for returning receive signals, on the other hand, decreases transmission speed substantially because primary communication is stopped during the return of data.